1. Field of the invention:
This invention relates to a semiconductor laser device. More particularly, it relates to a semiconductor laser device having a structure which is effective to control a transverse mode of laser oscillation, to lower the threshold current level and to increase the life span, and which is produced by the use of a crystal growth technique for the formation of ultra-thin films such as molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MO-CVD).
2. Description of the prior art:
Recently, a single crystal growth technique for the formation of thin films such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MO-CVD), etc., has been developed which enables the formation of thin film growth layers having a thickness of as thin as approximately 10 .ANG.. The development of such a technique, although these significantly thin films have not yet been produced by liquid phase epitaxy (LPE), allowed the thin films to be applied to lasers, resulting in laser devices exhibiting new laser effects and/or superior laser characteristics. A typical example of these new laser devices is a quantum well (QW) laser, which is produced based on the fact that quantization levels are established in its active layer by reducing the thickness of the active layer from several hundred 521 to approximately 100 .ANG. or less and which is advantageous over conventional double heterostructure lasers in that the threshold current level is low and the temperature and transient characteristics are superior. Such a quantum well laser is described in detail in the following papers:
(1) W. T. Tsang, Applied Physics Letters, vol. 39, No. 10, pp. 786 (1981), PA1 (2) N. K. Dutta, Journal of Applied Physics, vol. 53, No. 11, pp. 7211 (1982), and PA1 (3) H. Iwamura, T. Saku, T. Ishibashi, K. Otsuka, Y. Horikoshi, Electronics Letters, vol. 19, No. 5, pp. 180 (1983).
As mentioned above, the single crystal growth technique, such as molecular beam epitaxy or metal-organic chemical vapor deposition, has resulted in the practical use of high quality semiconductor lasers having a new multiple-layered structure. However, the semiconductor laser is deficient in that a stabilized transverse mode of laser oscillation cannot be attained due to its multiple-layered structure.
One of the most important points requiring improvement in other conventional semiconductor lasers which are in practical use is stabilization of the transverse mode of laser oscillation. A contact stripe geometric laser, which was developed in the early stage of laser development, has a striped electrode to prevent injected current from transversely expanding, and attains laser oscillation in a zero order mode (i.e., a fundamental transverse mode) upon exceeding the threshold current level due to the fact that gain required for laser oscillation is greater than losses within the active region underneath the stripe region, while the said contact stripe geometric laser produces laser oscillation inan expanded transverse mode or a higher-order transverse mode with an increase in the injection of current beyond the threshold current level, because carriers which are injected into the active layer spread to the outside of the striped region resulting in expanding the high gain region. Due to such an unstable transverse mode and dependency of the transverse mode on the amount of injected current, the linear relationship between the injected current and the laser output decreases. Moreover, the laser output resulting from pulse modulation is unstable so that the signal-noise ratio is reduced and its directivity becomes too unstable to be used in an optical system such as optical fibers, etc. In order to overcome the above-mentioned practical drawbacks of contact stripe geometric lasers, a variety of structures for semiconductor lasers of GaAlAs and/or InGaAsP systems have been already produced by liquid phase epitaxy, which prevent not only current but also light from transversely expanding thereby attaining stabilization in the transverse mode. However, most of these semiconductor lasers can only be produced by the growth of thin film layers on a channeled substrate, a mesa substrate or a terraced substrate based on a peculiarity of liquid phase epitaxy, typical examples of which are channeled substrate planar structure injection lasers (CSP lasers) (K. Aiki, M. Nakamura, T. Kuroda and J. Umeda, Applied Physics Letters, vol. 30, No. 12, pp. 649 (1977)), constricted double heterojunction lasers (CDH lasers) (D. Botez, Applied Physics Letters, vol. 33, pp. 872 (1978)) and terraced substrate lasers (TS lasers) (T. Sugino, M. Wada, H. Shimizu, K. Itoh, and I. Teramoto Applied Physics tters vol 34, No. 4, (1979)). All of these lasers can be only produced utilizing anisotrophy of the crystal growth rate, but they cannot be produced by the use of a crystal growth technique such as molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MO-CVD).
FIG. 3 shows a conventional GaAlAs semiconductor laser operating in a stabilized transverse mode, which is produced as follows: On an n-GaAs substrate 1, an n-GaAs buffer layer 1', an n-Ga.sub.0.7 Al.sub.0.3 As cladding layer 2, an n-GaAs active layer 3, a p-Ga.sub.0.7 Al.sub.0.3 As cladding layer 4, and a p-GaAs cap layer 5 are successively formed by molecular beam epitaxy, followed by subjecting to a vapor deposition treatment with metal materials of Al/Zn/Au in this order to form an electrode layer 25, which is then formed into a striped shape by photolithography. The semiconductor layers positioned outside of the striped electrode layer 25 is then eliminated by an Ar.sup.+ ion-beam etching technique using the striped electrode layer 25 as a masking material in such a manner that the thickness of the cladding layer 4 becomes approximately 0.3 .mu.m, resulting in an optical waveguide within the active layer 3 corresponding to the striped region 10. The electrode layer 25 is subjected to a heating treatment to be alloyed. On the cladding layer 4 outside of the striped region 10, a SiO.sub.2 film 6 and a p-sided Gr/Au electrode 8 are then formed. An n-sided AuGe/Ni electrode 7 is formed on the back face of the substrate 1, resulting in a laser device having relatively stabilized characteristics. However, this laser device is disadvantageous in that since the built-in refraction index difference of the optical waveguide depends upon precision of the depth of the semiconductor lasers to be etched by an Ar.sup.30 ion-beam etching technique, it is difficult to control the built-in refraction index difference, causing difficulty in obtaining a fundamental transverse mode oscillation with reproducibility, and/or that since a decrease in the refraction index difference is difficult, high output power cannot be created.
Moreover, this laser device has the significant drawback mentioned below: This laser device is mounted on a radiation plate of Cu, etc., by means of a soldering material such as In, etc., in order to improve heat-radiation of the laser device. However, the distance from the portions of the active layer 3 corresponding to the regions other than the striped region 10 to the mounting face of the radiation plate is as extremely small as 1 .mu.m or less, so that the active layer undergoes great stress due to thermal shrinkage based on a decrease in temperature after solidification of the soldering material, which makes the life span of the device short (T. Hayakawa et al., Appl. Phys. Lett., vol. 42, pp. 23 (1983)). In addition, the distance between the active layer and the mounting face corresponding to the striped region 10 is different from the distance therebetween corresponding to the regions other than the striped region 10, so that the active layer further undergoes great stress at the interface between these regions at the different distances, which accelerates deterioration of the device.